Recirculation valves are frequently utilized to cool centrifugal pumps. During periods of low downstream demand, the water in the centrifugal pump remains stationary. Over time, the stationary water can heat up due to the continuing operation of the pump. Recirculation valves operate to permit a constant circulation of fluid back to the pump during periods of low downstream demand.
Recirculation of the output from the pump can be provided by manual operation of the recirculation valve. This method is expensive and unreliable. Alternatively, recirculation can be provided by a bypass valve controlled by a flowmeter which detects flow in the inlet side of the pump. When the flow on the inlet side of the pump drops below the minimum required to cool the pump, the flowmeter opens the bypass valve, thereby maintaining the pump output at the required minimum, and diverting that portion of the flow not required by the feed heater to a sump connected to the inlet of the pump. Such an arrangement involves relatively complex and expensive apparatus and has not been found to be satisfactory. and expensive apparatus and has not been found to be satisfactory. An alternative to both of the techniques described above and one which has been found to be generally satisfactory for most systems, involves the use of a modulating flow control valve assembly including a main check valve, the position of which is responsive to the demand flow requirements of the heater and an on-off bypass valve for recirculating flow back to the pump during the periods of low demand by the heater.
While valves of the type described above are known in the art, they are not entirely satisfactory in certain systems, particularly in large steam generating plants with minimum flow requirements. One problem with valve assemblies of the type described above is caused by the on-off operation of the pump. Abrupt changes cause a water hammer effect, that is, sudden surges in the output of the pump which may be detrimental to the pump, the piping system, and the remainder of the associated fluid system.
U.S. Pat. No. 4,095,611, issued to Yarway Corporation of Blue Bell, Pa., and now assigned to Keystone International Holdings Corporation, the assignee of the present invention, is directed to a valve mechanism which provides modulating control. While the valve disclosed therein vastly improves over prior art apparatus, it incorporates a complex attachment mechanism to control the by-pass valve. Specifically, the by-pass valve is not directly coupled to the check valve assembly but rather is connected through a lever and piston arrangement. Accordingly, the valve is more expensive to repair and more difficult to replace. Second, the valve disclosed in this patent requires additional moving parts to control the movement of the recirculation valve mechanism which may impede the precision with which the by-pass valve can be controlled.
Due to the problems associated with by-pass valves containing mechanical linkages, pilot valves have also been utilized to control the flow of liquid through a recirculation valve. Pilot valves eliminate the need for linkages. U.S. Pat. No. 4,019,527 issued to Yarway Corp., and now also assigned to Keystone International Holdings Corp., assignee of the present invention, discloses such a device. The device disclosed in U.S. Pat. No. 4,019,527 utilizes a pilot valve to control the operation of the recirculation valve. While the pilot valve disclosed in U.S. Pat. No. 4,019,527 offers numerous advantages, the recirculation valve may stall at high pressures Water contained in the pilot valve piston chamber of U.S. Pat. No. 4,019,527 can only drain through two narrow horizontal flow conduits. Because of the relative positions of the check valve and the recirculation valve, the slow drainage of the recirculation valve may impede the downward movement of the check valve.
It is an object of this invention to provide a modulating recirculation flow control valve assembly which improves over U.S. Pat. No. 4,095,611 and which includes a check valve and bypass flow control valve which are coupled such that the flow through the bypass valve varies inversely with the flow through the check valve.
It is a further object of this invention to provide a modulating flow control valve assembly including a check valve and bypass flow control valve which responds rapidly.
It is still a further object of the present invention to provide a recirculation valve which provides for modulating by-pass control through a novel pilot valve arrangement.
It is an additional object of this invention to provide a recirculation valve controlled by a pilot which can drain quickly and which will not impede the movement of the check valve.
It is an additional object of the present invention to provide a pilot valve controlled recirculation valve in which the motion of the check valve is not impeded by the relative position of the recirculation valve.
It is yet a further object of the present invention to provide a recirculation valve having a pilot valve with variable switch point.
It is yet an additional object of this invention to provide a modulating flow control valve assembly which is simple, rugged, economical, and which is particularly useful in steam generating plants in order to maintain a minimum output from a feed water pump.
It is still yet a further object of the present invention to provide a recirculation valve which can easily be inserted, adjusted, repaired and replaced.
It is a further object of the present invention to provide a recirculation valve which can function in either a modulating or on/off configuration.